1. Field of the Invention
This invention generally relates to improvements in papermakers belts. More particularly, the invention relates to the flame-proofing of a resin-treated dryer fabric.
In the papermaking operation, the newly formed paper web must be dried after initially pressing it, in completion of the removal of water. As the paper web passes through the dryer section of a papermaking machine, it is guided throughout its passage by either a single or a pair of fabrics supported on rolls and known as dryer felts or fabrics. The paper web passes between the felts, and the dryer cylinders and exits the dryer unit. One of the great dangers in all papermaking operations is that of fire. The danger of fire is especially acute in the drying operation, due to the heating involved, and the start-up and shut-down involved. There is an ever-present danger that the drying fabrics, because they remain in the dryer and are continuously subjected to heat, will catch fire. Another possible cause of fire is static discharge, where a spark may ignite the dust or paper fines in the atmosphere or on the fabric. One approach to minimize fabrics catching fire is to modify the finished fabric to make it more difficult to catch fire, and if it does, then also to make the fabric more rapidly self-extinguishing.
In regard to the flame retardant properties of fabrics, it is well known that blends or combinations of materials will tend to be more flammable than their individual components. For example, a fabric, woven from thermoplastic yarns such as polyester, is relatively slow to propagate flame and is often self-extinguishing because the molten polymer constituting the yarn withdraws from the flame and/or melts and drips away, thereby removing the flame from the burning zone.
On the other hand, once fiber blends are employed in a fabric, the less fire resistant material burns, while the more fire resistant material acts as a scaffold preventing the withdrawal, dripping or falling away of the less fire resistant material. This is known as the scaffolding effect, and it is primarily found in blends and combinations of yarns.
Generally, when a fabric is resin-treated, the non-melting resin acts as a scaffold and the total fabric burns vigorously. Because the scaffolding effect was not originally clearly recognized, it was thought that flame-proofing of the resin would prevent or inhibit burning of resin-treated fabrics; however, such was not the case. Once the scaffolding effect was recognized, it seemed reasonable to attempt to flame-proof the basic material, such that, even if the resin were present, the basic material would not burn. This approach was also not successful, because the scaffolding effect continued to work, but in reverse, with the basic fabric material (now flame-proofed) acting as the scaffold. Thus, a flame-proof polyester fabric prior to resin coating would not burn, whereas after resin coating, the composite burned, with the fabric acting as the scaffold and the resin burning slowly.
With regard to the flame-proofing materials in general, the use of phosphorous, antimony oxide, and brominated compounds as flame retardants has been known for some time. However, the basic application of flame-proofing has been in the apparel or clothing art where a resin is added to prevent the removal of the flame retardant during laundering. Thus, for example, resins such as polyurethane, latex or chlorinated paraffin were found suitable for addition to fabrics which had been flame-proofed to aid in retention of fire proof properties. It was also known to flame-proof polyester fibers by incorporating distinct phosphorous compounds, for example, phosphorates or certain diphosphonic acids into the chain molecules. Further, flame retardant methods applicable to fiber blends were based upon additives being incorporated in the individual components of the blend.
However, the prior art fire-proofing was not developed in the direction of resin-treated dryer fabrics, and thus, did not provide a solution to the above-mentioned problem with regard to the burning of resin-treated dryer fabrics. Further, in the prior art dealing with clothing or apparel, the resin is of secondary importance as it is merely an aid to retention of the fire retardant agent in the fabric, and therefore the resin is generally chosen to be compatible with the flame retardant; whereas in the case of dryer fabrics, the resin is of primary importance for imparting fabric stability, wear and abrasion resistance, heat and hydrolysis resistance, resistance to chemical attack, oil and dirt resistance, and modulus properties to the fabric. There is thus a need for a resin-treated dryer fabric exhibiting flame-proof characteristics. There is also a need for providing a solution to the flame-proofing problem peculiar to resin-treated dryer fabrics. The present invention is directed toward filling those needs.